package leetcode.秋招.广;

import java.util.Scanner;

/**
 * @author ：zsy
 * @date ：Created 2021/9/17 21:37
 * @description：
 */
public class Main4 {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        int n = in.nextInt();
        System.out.println(1 + n + n * (n - 1) / 2);
    }
}
